Electrical hygrometers



ELECTRICAL HYGROMETERS Filed June 21, 1960 F ig. 2.-

United States Patent 3,033,573 ELEC'fRlCAL HYGRQMETERS John L. haw, 31 Market St., Bradford, England Filed June 21, 196i}, Ser. No, 37,809 Claims priority, appiieation Great Britain June 23, 1959 6 Claims. (Cl. 73-6365) The invention relates to hygrometers and is more particularly concerned with humidity-sensitive elements for hygrometers.

The invention provides a humidity-sensitive element for a hygrometer, which element comprises a metal base (preferably of aluminium) provided with a surface layer (preferably an electrically anodized layer) having pores or capillaries, or the like, which extend partly through it from its outer face and are partly filled with an aqueous solution of a hygroscopic substance, which layer is covered by a water-vapour-permeable layer of electrically conductive material (e.g. graphite or vacuum evaporated metal), whereby the electrical capacitance between the base and the water-permeable electrically conductive layer varies with the humidity to which the water-permeable layer is exposed. The said aqueous solution of said hygroscopic substance is preferably applied as a saturated solution.

The aqueous solution which partly fills the pores gains or loses water by passage of water vapour through the water-vapour-permeable layer until the vapour pressure at the mouths of the pores approaches equilibrium with that outside of the water-vapour-permeable layer, so that the amount of solution in the pores, when such equilibrium has been substantially reached, is a measure of the humidity. The electrical capacitance between the watervapour-permeable layer and the metal base varies with the amount of solution in the pores and consequently that capacitance varies with the humidity.

The variation of the electrical capacitance in accordance with the humidity can be measured or indicated by any suitable means, which may be calibrated in absolute or relative humidity.

The aforementioned pores or capillaries are formed automatically during the anodizing process, but it may be necessary to try variations of the anodizing conditions to obtain best results.

The aqueous solution may be introduced into the pores in the necessary quantity by applying a very thin layer of the saturated solution to the outside of the watervapour-permeable layer if that layer is also water-permeable, in which case the solution permeates that layer, but preferably it is done by applying a very thin layer of the saturated solution to the outer face of the anodized layer before the water-vapour-permeable layer is applied. The necessary thickness of the layer of saturated solution to achieve partial filling of the pores is of the order of a few microns.

The invention includes a method of manufacturing a humidity-sensitive element for a hygrometer, which method includes the steps of anodizing a metal base to form an anodized layer thereon, covering the anodized layer with a water-vapour-permeable electrically conducting layer, and applying a very thin layer of an aqueous solution of a hygroscopic substance either to the outer face of the water-vapour-permeable layer, if that layer is also Water-permeable, or to the outer face of the anodized layer before it is covered by the water-vapour-permeable layer.

The particular hygroscopic substance employed is preferably chosen in accordance with the humidity conditions which are to be measured or indicated. Lithium chloride is particularly useful for the purpose and enables elements to be made which can be used with humidity conditions at 20 C. having dew points between l120 C. and -20 C. Use of calcium chloride enables elements to be made which can be used with humidity conditions at 20 C. having dew points between -70 C. and 0 C. Chromium oxide is suitable for use in elements which are intended for use in relative humidities in the range 2-40%. The hygroscopic substance used must of course be one which in fact goes into solution in the water, so that some of the water can escape from the solution when the humidity falls, and not one which combines chemically with the water and prevents such escape.

The nature of the invention and the manner of carrying the same into effect may be more clearly understood by the aid of the accompanying drawings, wherein:

FIGURE 1 illustrates, by a magnified partial cross section, one example of construction of an element according to the invention; and

FIGURE 2 illustrates one example of an electrical circuit for use with the invention.

As shown in FIGURE 1, the element consists in an aluminium wire a which is anodized in a 3% solution of oxalic acid at a potential of volts for a period of 30 minutes to produce an anodized layer 12 containing a multiplicity of pores or capillaries 0. After anodizing, the wire is washed to remove traces of the acid. Next a very thin layer d of an aqueous solution of lithium chloride is painted over the anodized surface so as to partly fill the pores, this can be done by dipping a dry camel hair brush into the solution, squeezing out the surplus liquid, and lightly brushing over the anodized surface. The outer electrically conducting layer 2 is then applied in a high vacuum and consists of 24 carat gold evaporated onto the anodized layer.

An element 5 cms. long and of 20 s.w.g. Wire constructed as above. described has a capacitance of the order of 0.02 microfarad.

FIGURE 2 shows one example of an electrical circuit with which an element constructed as above described can be used to indicate low dew points. This consists of a capacitance bridge 1 operating at 50 c./s. or 60 c./s. and having its terminals connected respectively to the aluminium wire a and its outer conducting layer 2. The change in capacitance observed on the bridge is then proportional to the humidity surrounding the element.

I claim:

1. A humidity-sensitive element for a hygrometer, comprising an aluminium base provided with an oxidized porous surface layer of aluminium oxide, said surface layer having pores which extend partly through it from its outer face, said pores being partly filled with an aqueous substantially saturated solution of a hygroscopic substance, and immediately over said liquid-treated layer a continuous covering consisting of a water-vapour-permeable layer of electrically conductive material whereby the electrical capacitance between the base and the waterpermeable electrically conductive layer varies with the humidity to which the Water-permeable layer is exposed, said partially filled pores permitting the release and picking up water vapor to the liquid in the pores by-the vapor transmitted through said covering layer.

2. A humidity-sensitive element according to claim 1, wherein said surface layer is constituted by an electrically anodized layer.

-3. A humidity-sensitive element according to claim 1, wherein said hygroscopic substance is lithium chloride.

4. A humidity-sensitive element according to claim wherein said hygroscopic substance is calcium chloride.

5. A humidity-sensitive element according to claim wherein said hygroscopic substance is chromium oxide.

6. A method of manufacturing a humidity-sensitive element for a hygrometer, which method includes the steps of anodizing an aluminium metal base to form a thin porous aluminium oxide layer thereon, applying to the surface of saidanodizedlayer a very layer of the order of microns of an aqueous substantially saturated splutiqn of a; hygroseq ic sulbstance to partly fill the pores of tihe enodiz ed ayer and thereafter hovering the 1i1uidi tqd nam ngweewramy fill d; t aq e u y r x e' uq i l e h, r a ex-v n a-nermca e tr e ll' w sllletin r e ea whea bx the e e r ee l apae ai qb twea e l e nd. he Ma e-Permeable. ele ically conductive layer varies with the humidity to which the Water-permeable layer-is exposed;

References Cited in the file of this patent UNITED SIATES PATENTS 

1. A HUMIDITY-SENSITIVE ELEMENT FOR A HYGROMETER, COMPRISING AN ALUMINIUM BASE PROVIDED WITH AN OXIDIZED POROUS SURFACE LAYER OF ALUMINIUM OXIDE, SAID SURFACE LAYER HAVING PORES WHICH EXTEND PARTLY THROUGH IT FROM ITS OUTER FACE, SAID PORES BEING PARTLY FILLED WITH AN AQUEOUS SUBSTANTIALLY SATURATED SOLUTION OF A HYGROSCOPIC SUBSTANCE, AND IMMEDIATELY OVER SAID LIQUID-TREATED LAYER A CONTINUOUS COVERING CONSISTING OF A WATER-VAPOUR-PERMEABLE LAYER OF ELECTRICALLY CONDUCTIVE MATERIAL WHEREBY THE ELECTRICAL CAPACITANCE BETWEEN THE BASE AND THE WATERPERMEABLE ELECTRICALLY CONDUCTIVE LAYER VARIES WITH THE HUMIDITY TO WHICH THE WATER-PERMEABLE LAYER IS EXPOSED, SAID PARTIALLY FILLED PORES PERMITTING THE RELEASE AND PICKING UP WATER VAPOR TO THE LIQUID IN THE PORES BY THE VAPOR TRANSMITTED THROUGH SAID COVERING LAYER.
 6. A METHOD OF MANUFACTURING A HUMIDITY-SENSITIVE ELEMENT FOR A HYGROMETER, WHICH METHOD INCLUDES THE STEPS OF ANODIZING AN ALUMINIUM METAL BASE TO FORM A THIN POROUS ALUMINIUM OXIDE LAYER THEREON, APPLYING TO THE SURFACE OF SAID ANODIZED LAYER A VERY THIN LAYER OF THE ORDER OF MICRONS OF AN AQUEOUS SUBSTANTIALLY SATURATED SOLUTION OF A HYGROSCOPIC SUBSTANCE TO PARTLY FILL THE PORES OF THE ANODIZED LAYER AND THEREAFTER COVERING WITH AQUEOUS TREATED ALUMINIUM OXIDE LAYER PARTLY FILLED WITH AQUEOUS HYGROSCOPIC SUBSTANCE WITH A WATER-VAPOUR-PERMEABLE ELECTRICALLY CONDUCTING LAYER WHEREBY THE ELECTRICAL CAPACITANCE BETWEEN THE BASE AND THE WATER-PERMEABLE ELECTRICALLY CONDUCTIVE LAYER VARIES WITH THE HUMIDITY TO WHICH THE WATER-PERMEABLE LAYER IS EXPOSED. 